The invention relates to coherent optical sources, particularly solid state lasers.
Lasers operating on the principle of a structure with multiple parallel optical waveguides coupled laterally with each other (multiple optical waveguide structure) have already been devised.
In these structures, if the optical waveguides are placed side by side, very close to each other, there is a mutual coupling of the optical wave travelling in one guide with the optical wave of neighbouring guides. It is therefore possible by making use of the mutual coupling between guides, i.e. in practice by correctly choosing the distances between neighbouring guides, to obtain at the output optical waves that are coherent, with a distribution of electric field, and therefore of light energy, which is not the simple superposition of the electric fields produced in each guide but which is a composition corresponding to a mode of propagation characteristic of the multiple guide structure.
Consequently, whereas uncoupled juxtaposed guides would lead to a distribution of light energy distributed uniformly between the outputs of the different guides, mutual optical coupling can lead to a non-uniform energy distribution, possibly concentrated at the centre, for example with a shape that is approximately Gaussian. Such a structure is thus very worthwhile since it enables the energy from a spatially distributed output to be concentrated.
Unfortunately, experiment shows that practical realization is very difficult. In effect, it is very difficult to control the dimensions and homogeneity of optical waveguides, and nobody has actually succeeded in making a laser operate in this way, with a beam really possessing an overall natural mode produced from the individual modes of the different guides. A correct concentration of energy is therefore not obtained.
An attempt has been made to achieve this, for example with semiconductor lasers, in which the emission of light is stimulated by the injection of currents into the junctions. However, it is realised that the injection of current alters the refractive indices in the guides. These changes in index modify the phases of the optical waves. There is a failure to obtain in the different guides a distribution of phases which would combine to give a beam intensity distributed spatially in the desired manner.